This invention relates to thin film solid state electrochemical cells. More particularly, this invention relates to novel positive electrodes consisting of composites of an organo-sulfur compound, a p-doped conjugated polymer and a polymer electrolyte.
Considerable interest has been shown over the last few years in thin film secondary lithium batteries based on polymer electrolytes and insertion material cathodes. The intense research effort is driven by new requirements for high performance batteries in portable communication equipment, consumer electronics, portable computers and electric vehicles. Thin film solid state cells using cathodes based on organo-sulfur compounds are particularly well suited because of their high energy to weight ratio.
Two main types of cathode materials used in the manufacture of thin film lithium and sodium batteries are known in the art. The first material includes transition metal chalcogenides, such as titanium disulfide with alkali metals as the anode. For example, among the cathode active chalcogenides, U.S. Pat. No. 4,049,879 lists transition metal phosphorous chalcogenides. Other U.S. Pat. Nos. such as 4,143,214, 4,152,491 and 4,664,991 describe cells wherein the cathode is a carbon/sulfur based material, generally of the C.sub.x S formula where x is typically 10 or larger.
U.S. Pat. No. 4,142,294 to Chang, et al. describes cells having cathodes containing C.sub.x S, wherein x is a numerical value from about 4 to about 50. U.S. Pat. No. 4,152,491 to Chang, et al. relates to electric current producing cells where the cathode-active materials include one or more polymer compounds having a plurality of carbon monosulfide units. The carbon monosulfide unit is generally described as (CS).sub.x wherein x is an integer of at least 5, and may be at least 50, and is preferably at least 100. In both cells developed by Chang et al., the energy storage capacity is limited because there is a low density of sulfur-sulfur bonds.
U.S. Pat. No. 4,664,991 to Perichaud, et al. describes a substance containing a one-dimensional electric conducting polymer and at least one polysulfurated chain forming a complex with the polymer. Perichaud, et al. use a material which has two components. One is the conducting polymer, which is selected from a group consisting of polyacetylenes, polyparaphenylenes, polythiophenes, polypyrroles, polyanilines and their substituted derivatives. The other is a polysulfurated chain which is in a charge transfer relation to the conducting polymer. The polysulfurated chain is formed by high temperature heating of sulfur to produce a chain of the composition . . . S-S-S-S . . . of indeterminate length. As a result of using this material, the cell of Perichaud, et al, exhibits a fairly low voltage of only 2.0 volts against lithium.
In a related approach, U.S. Pat. No. 4,833,048 to DeJonghe, et al. describes a cathode made of organo-sulfur compounds of the formula (R(S).sub.y).sub.n where y=1 to 6; n=2 to 20, and R is one or more different aliphatic or aromatic organic moieties having one to twenty carbon atoms. One or more oxygen, sulfur, nitrogen or fluorine atoms associated with the chain when R is an aliphatic chain can also be included. The aliphatic chain may be linear or branched, saturated or unsaturated. The aliphatic chain or the aromatic rings may have substituted groups. The preferred form of the cathode material is a simple dimer or (RS).sub.2. When the organic moiety R is a straight or branched aliphatic chain, such moieties as alkyl, alkenyl, alkynyl, alkoxyalkyl, alkythioalkyl, or aminoalkyl group and their fluorine derivatives may be included. When the organic moiety comprises an aromatic group, the group may comprise an aryl, aralkyl or alkylaryl, including fluorine substituted derivatives, and the ring may contain one or more nitrogen, sulfur or oxygen heteroatoms in the ring as well.
In the cell developed by DeJonghe, et al., the main cathode reaction during discharge and recharge of the cell, is the breaking and reforming of sulfur-sulfur bonds. The breaking of sulfur-sulfur bonds is associated with the formation of sulfur-metal ionic complexes. The organo-sulfur materials investigated by DeJonghe, et al. undergo polymerization and depolymerization upon the formation and breaking of the sulfur-sulfur bonds. The depolymerization which occurs during the discharging of the cell results in lower weight monomeric species which can dissolve into the electrolyte. The result is an unsatisfactory cycle life having a maximum of about 200 discharge-charge cycles.
Sotomura et al. [Electrochimica Acta, Vol. 37, pp. 1851-1854, (1992)], discusses the formation of composite cathode materials for lithium cells based on conducting polyaniline, a gel polymer electrolyte and a 2,5-dimercapto-1,3,4-thiadiazole compound which can undergo polymerization-depolymerization reactions as described by DeJonghe et al. The composite with polyaniline provides a higher voltage vs. lithium anodes than can be provided by the organo-sulfur compound alone. The key problem, as revealed by the compounds described by DeJonghe et al., remains, however, as the organo-sulfur compound depolymerizes upon discharge of the cell to form monomeric species which diffuses into the bulk of the electrolyte and severely limits the cycle life of the cell.
Despite the various approaches proposed for organo-sulfur cathode materials, there remains a need for an inexpensive cathode material having a high storage capacity, high discharge rate and a long cycle life at ambient temperatures.
Whereas the organo-sulfur materials described by DeJonghe et al. and by Sotomura et al. lead to the formation of monomeric species upon discharge of the cell, and consequently, a severely limited cycle life due to diffusion of the monomeric cathode materials into the bulk electrolyte, it is a primary objective of this invention to provide new composite cathode materials which avoid the limitations existing in the prior art, while offering performance characteristics much higher than those of known materials, and having an actual specific energy in excess of 1,000 Wh/kg.
It is another object of this invention to provide a new composite cathode having as the active material an organo-sulfur compound and a p-doped conjugated polymer as the electronically conducting component, wherein the organo-sulfur compound is a high molecular weight compound which does not undergo de-polymerization to form soluble molecular species upon reduction.
It is another object of this invention to provide a new composite cathode having as the active cathode material poly(carbon disulfide) having the formula (CS.sub.x).sub.n where x is from about 1.7 to about 2.3, and where n is greater than 2, and preferably greater than 20, and most preferably greater than 50; and p-doped polyaniline (polyaniline) as the electronically conducting component.
It is yet another object of this invention to provide a method of making a solid state rechargeable cell including the novel composite cathode of the invention.
Co-pending U.S. Pat. application Ser. No. 07/991,948, filed on Dec. 17, 1992, describes the use of poly(carbon disulfide) as cathode active material. Reference is made thereto, and is incorporated herein.